The invention relates to an arrangement for particle beam measurement in which primary particles are imaged on a specimen and a spectrometer serves to detect secondary particles being controllably focused.
In the measurement of electric potentials with an electron probe, a potential resolution can be obtained which is given by the formula: ##EQU1## This potential resolution becomes better the greater the current i.sub.s of the electron probe, if the spectrometer constant k, the pulse-duty factor p, and the bandwidth .DELTA.f are specified.
There are conventional lens errors in the objective or focus field forming the electron probe, such that the probe current i.sub.s is dependent upon these conventional lens errors. By way of approximation, this connection is indicated by the formula: EQU i.sub.s =1,13.pi.R.alpha..sup.2 {d.sup.2 -[(2C.sub.F .DELTA.U.alpha./U).sup.2 +(C.sub.O .alpha..sup.3 /2).sup.2 ]}
It is apparent that, with a given diameter d of the electron probe, a given radiated beam value R, a given aperture .alpha. and a given energy width .DELTA.U of the electron probe, the current i.sub.s of the electron probe becomes smaller the greater the axial chromatic aberration C.sub.F and the aperature error C.sub.O of the lens of the objective forming the electron probe become. The current i.sub.s of the electron probe can be effectively increased if the lens aberrations are reduced by means of a greater lens current with which the lens is excited. On the other hand, the focal length of this lens and hence the working distance is thereby shortened so that there may be insufficient room for the spectrometer detector arrangement between specimen and objective lens.
A potential measurement technique has previously been adapted for conventional scanning electron microscopy. A spectrometer for the quantitative potential measurement is applied between the specimen and the object lens as set forth by H. P. Feuerbaum in Scanning Electron Microscopy (1979), pp. 285-296 or in the U.S. Pat. Nos. 4,277,679 and 4,292,519, respectively. Such a known spectrometer has a minimum working distance which, at least, must be afforded. Hence, reduction of the objective lens aberrations, causing a shortening of the focal length of this objective lens and hence of the working distance, is not readily possible in this previously known technique for particle beam measurement.
The invention concerns an arrangement in which probe current can be effectively increased and/or permits an improved potential resolution for use with the above-described particle beam measurement technique. The invention enables objective lens arrangements with an integrated spectrometer in which objective lens aberrations are decreased by means of shorter focal lengths. Given the same probe current i.sub.s, an improved local resolution as well as an improved potential resolution can thus be achieved.